Goals: When Mariner 8 suffered a failed launch, Mariner 9 took on that spacecraft's mission of mapping 70 percent of the Martian surface in addition to its own objective of studying changes over time in the Martian atmosphere and surface.

Accomplishments: Mariner 9 beat the Soviet Mars 2 (which had an 11-day head start) to Mars, becoming the first spacecraft to orbit another planet. After Mars' global dust storm subsided, the orbiter was able to map 85 percent of the Martian surface at a resolution of 1 to 2 km (with 2 percent mapped at a resolution of 100 to 300 meters) in addition to gathering abundant information about the surface and atmosphere. The more than 7,000 images it transmitted included the first detailed views of Olympus Mons, the solar system's largest volcano; Valles Marineris, a vast canyon system that dwarfs the Grand Canyon and was named for this spacecraft; the polar caps; and the moons Phobos and Deimos.